In a typical wireless communication network, wireless devices, also known as wireless communication devices, mobile stations, stations (STA) and/or user equipments (UE), communicate via a Radio Access Network (RAN) to one or more core networks (CN). The RAN covers a geographical area which is divided into areas or cell areas, with each area or cell area being served by an access node e.g., a Wi-Fi access point or a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. The area or cell area is a geographical area where radio coverage is provided by the access node. The access node communicates over an air interface operating on radio frequencies with the wireless device within range of the access node.
A Universal Mobile Telecommunications System (UMTS) is a third generation telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several access nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural access nodes connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising radio base stations connected directly to one or more core networks.
Due to recent technology and standardization developments, introducing large antenna arrays at cellular base stations and other wireless access points has become a viable option to boost the capacity and user data rates in the wireless communication network. A base station (BS) or an access point (AP) equipped with excessive number of antennas, can simultaneously schedule multiple wireless devices at the same time or frequency band with simple linear processing such as maximum-ratio transmission (MRT) or zero-forcing (ZF) in a downlink (DL) and maximum-ratio combining (MRC) or ZF in an uplink (UL). This is often referred to as very-large, or full dimension (FD), multiple-input multiple-output (VL-MIMO) or massive MIMO in the literature. The gains with VL-MIMO are achieved without consuming any additional spectrum. Additionally, the radiated energy efficiency with VL-MIMO can be substantially improved. Recognizing the technology potential, the 3GPP has defined a work item on Full Dimension (FD) MIMO.
A key usage of FD MIMO technology is extreme narrow beam forming for DL transmissions, that enables a BS to focus the transmitted energy to the desired wireless device and thereby boosting the coverage and user data rates for DL transmissions.
For FD MIMO systems it is not trivial how to acquire Channel State Information (CSI) in a scalable fashion, which is essential to gain the performance potentials of excessive amount of transmit antennas. Traditionally, each wireless device continuously measures on pilot, or reference, symbols transmitted by the BS during downlink transmission phase, to estimate the downlink channel gain and feeds it back to the BS via a reverse link.
Since the number of required pilots in the downlink is proportional to the number of BS antennas, feedback based schemes are not scalable. Existing solutions to this problem operate in the time-division duplex (TDD) mode and rely on the channel reciprocity between the uplink and the downlink. More precisely, each wireless device transmits sounding reference signals (SRS) in the uplink phase which are then used by the BS to estimate both the uplink and downlink wireless channel. The number of uplink pilots in these schemes is proportional to the number of wireless devices, which are typically smaller than the number of BS antennas.
In existing systems, wireless channel sounding, also called wideband channel sounding, refers to the mechanism that enables an access point or BS to obtain wideband channel state information in parts of the spectrum, in which currently no wireless data transmission is taking place. Specifically, in cellular systems, a BS has two main usages of wideband channel sounding:                To acquire UL channel state information in frequency and time resources in which a wireless device is currently not scheduled, even though the wireless device may currently use other parts of the spectrum;        To acquire UL channel state information of wireless devices that are currently not transmitting uplink data.        
Licensed Assisted Access (LAA) and the standards development of LTE will enable to deploy LTE wireless access points and BSs operating in unlicensed or lightly licensed spectrum bands. In such deployments, the transmit power both at the BS side and the wireless device side will be confined to regulatory constraints, affecting the range of both the UL and DL transmissions.
In contrast to currently deployed cellular and wireless systems, in which CSI for DL transmission is obtained by feedback signaling of measurement on DL reference signals, in future full dimension and very large MIMO systems, such CSI information must be acquired based on UL reference signals transmitted by end user wireless devices. To ensure a high quality CSI estimate, uplink SRS transmissions by wireless devices need to be received with a sufficiently high Signal-to-Noise-Ratio (SNR) at the BS.
Given that the channel should be sounded sufficiently often with respect to the coherence time of the channel, over the entire frequency band, including possibly some unlicensed bands in case of LAA significant amount of energy needs to be radiated by each wireless device for the uplink reference or pilot transmissions, e.g. SRS transmissions. Since the wireless devices are typically battery-limited devices, channel sounding poses both a coverage and an energy efficiency problem for wireless devices, especially those that are in the cell edge.
Thus, there is a problem relating to energy inefficiency and coverage limitation of uplink channel sounding due to existing solutions in, e.g. VL MIMO, systems resulting in a limited or reduced performance of the wireless communication network.